A fiber in metal tube (FIMT), which is an optical fiber sheathing metal tube, has been conventionally used in a variety of fields including a radiation thermometer because having advantages such as in that it can sheathe a plurality of optical fiber elements and exhibits a good sealing performance against water or hydrogen gas, and has a necessary mechanical strength such as tensile strength without reinforcement, and further because having capability of a temperature sensor and being able to measure a long-distance continuous temperature distribution along the optical fiber due to the fact that the optical fiber in the FIMT is subject to no force by pressure.
As an example of the fiber optic cable using such an FIMT as described above, there is disclosed a fiber optic cable that is made up of a central copper conductor; an inner sheath made of low density polyethylene and cylindrically surrounding the conductor; a first layer formed of pluralities of steel wires and FIMTs using a stainless steel tube of the same diameter, and wound helically about the inner layer; a second layer of a plurality of steel wires wound around the first layer helically in the direction opposite to the first layer; and an outer layer formed of medium density polyethylene and surrounding the second layer (see, for example, Patent Document 1).
Also, there is recently proposed a fiber optic cable as another example of using an FIMT. The fiber optic cable is made up of a first optical fiber arranged at the center and directly exposed to the outside environment to measure a pressure distribution along the axial (longitudinal) direction of the optical fiber cable, not only a temperature distribution; a first layer formed of a plurality of metal wires and a stainless steel tube accommodating a second optical fiber, and surrounding the first optical fiber; and a second layer formed of a plurality of metal wires and surrounding the first layer, wherein at least one of the plurality of metal wires in the first layer and at least one of the plurality of metal wires in the second layer have their outer diameter reduced at intervals in the longitudinal direction, for the first optical fiber to be exposed to the pressure from oil or gas (see, for example, Patent Document 2).
There is further disclosed a fiber optic cable used in a distributed fiber optic system for measuring distributions of temperature, pressure, and strain. The fiber optic cable is formed to have a constant clearance space between cylindrical shaped multilayer armored wirers formed of a plurality of steel wires and a sensor optical fiber sheathing metal tube, to further improve the accuracy in measuring strain of a measurement target with the sensor optical fiber, wherein in order to form the constant clearance space, the clearance space between the sensor optical fiber sheathing metal tube and the multilayer armored wirers are filled with a water-dissolvable resin or an oil-dissolvable resin in an early step of the manufacturing the fiber optic cable, and the water-dissolvable resin coat or the oil-dissolvable resin coat is removed by immersing the fiber optic cable in water or oil in a later step thereof, and then an epoxy resin or the like is injected into the fiber optic cable at appropriate intervals along the axial (longitudinal) direction thereof, to fix the sensor optical fiber sheathing metal tube and the multilayer armored wirers to each other (see, for example, Patent Document 3).